make -d:nimFpRoundtrips work consistently in vm vs rt, fix #18400, etc

[timotheecour]

• vmops now supports var params
• -d:nimFpRoundtrips and -u:nimFpRoundtrips now works consistently in RT and VM
• fix all inconsistency issues mentioned in make dragonbox opt-in via -d:nimFpRoundtrips #18504 (comment)
• after this PR, roundtrip floats can be enabled in VM without having to bootstrap compiler (whereas before this PR, including when dragonbox was introduced, you would've needed bootstrapping via koch boot instead of simply nim c -o:bin/nim compiler/nim); this is a subtle point, i can explain more if needed
• fix schubfach & dragonbox & dollar bug #18400
• fix backticks : Float-like part of a variable identifier is changed in [XDeclaredButNotUsed] Hint #14407 (which was technically un-fixed by make dragonbox opt-in via -d:nimFpRoundtrips #18504)

other (but related)

• the code is now cleaner, mFloatToStr was removed, all the float to string logic was moved to formatfloat instead of being spread over many places
• lots of tests added to tests/float/tfloats.nim
• cgen now generates roundtrip literals instead of using c_snprintf(result.cstring, result.len.uint, "%#.16e%s", f, literalPostfix.cstring) in toStrMaxPrecision

note

I haven't change the default (introduced in #18504) but changing the default should now be a 1-liner to be put in config/config.nims